多组分钼铋系催化剂的还原机理

Mechanism of reduction of a multicomponent molybdate catalyst

通过全反射X射线荧光光谱分析(TXRF)和X射线衍射法(XRD)分析新鲜催化剂(MB98)的元素组成,主要元素以Bi2Mo3O12,Bi2MoO6,FeMoO4,CoMoO4,NiMoO4,MoO3和Fe2Mo3O12的形式存在。通过XRD、拉曼光谱(Raman)和光电子能谱法(XPS)分析多组分钼铋系催化剂的还原机理,测定不同还原程度催化剂中Mo,Bi,Fe,Co,Ni的主要物相结构和离子价态分布。XRD和Raman测试结果表明:催化剂还原过程分4步反应序贯进行;Fe2Mo3O12和(Fe/Co/Ni)MoO4中的晶格氧通过体相扩散向钼酸铋迁移;催化剂的储氧能力不仅与钼酸铋有关,还与Fe2Mo3O12和(Fe/Co/Ni)MoO4有关。XPS测试结果表明:还原生成的FeMoO4,MoO2和Bi主要分布于催化剂体相;在还原过程中,元素Mo,Bi首先在催化剂表面富集,然后向体相迁移;元素Fe,Co,Ni由体相向表面迁移。

The elemental composition of a fresh multicomponent molybdate catalyst （MB98） was studied using Total Reflection X-ray Fluorescence （TXRF） and X-ray powder diffraction （XRD） methods. The results showed that the key elements are present as Bi2Mo3012, Bi2MoO6, FeMoO4, CoMoO4, NiMoO4, MoO3 and Fe2Mo3012. The mechanism of reduction of the multicomponent molybdate catalyst and the structure and surface properties of the catalysts with various degrees of reduction were studied by XRD, laser Raman spectroscopy （Raman）, .and X-ray photoelectron spectroscopy （XPS） methods. The XRD and Raman results showed that four consecutive reactions occur during the reduction process and that lattice oxygen migrates from iron/cobalt/ nickel molybdates to bismuth molybdate through bulk diffusion. The oxygen storage capacity of the catalyst is not only related to the content of bismuth molybdate, but also to the content of FeaM03012 and （Fe/Co/Ni） MOO4. The XPS results showed that FeMoO4, MoO2 and Bi are mainly present in the bulk of the catalyst. Mo and Bi are concentrated on the surface and then migrate from the surface to the bulk. Fe, Co and Ni migrate from the bulk to the surface during the reduction reaction